The invention relates to a method of and apparatus for utilization of Laser Raman Spectroscopic technology for identification of material inclusions in a cellulose matrix, particularly the identification and quantification of sclereids, shives and/or stickies that may be present in pulp or paper.
Sclereids are dense cellulosic inclusions or stone cells occurring in both hardwood and softwood fibers. When embodied in papermaking pulp in sufficient numbers, sizes and/or concentrations, they can cause a variety of problems in papermaking, calendering, coating and converting operations. In the finished paper, they may produce blemishes, reduce the visual quality of the paper, result in non-uniform reception of printing inks, etc. There are few tests available for measuring sclereid inclusions in pulp and paper and they are mainly empirical, based on human observation and manual count.
Shives are undesireable wood fiber particles that are occasionally present in a finished paper product. Shives can have disadvantageous effects on the appearance, surface smoothness, ink receptivity and other aspects of a finished paper product. Few, if any, tests are available for accurate and efficient detection of shives in pulp and paper.
Stickies are encountered in the manufacture of papers made in whole or in part from recycled paper stock due to the presence of material inclusions in the recycled stock, for example, thermoplastics, pressure-sensitive adhesives, hot melts and wax. Under the heat and pressure of the papermaking process, these materials become tacky and adhere to various components of the papermaking machinery, such as wires, felts, dryers, calendars and coaters; thus the name stickies. Stickies impair paper machine runnability, cause web breaks, are one cause of coater blade streaks, and necessitate down-time for cleaning. They also create problems in converting operations and reduce the visual quality of the finished sheet of paper. With regard to the finished sheet, it is important for recycled-paper producers and merchants to identify and quantify these materials and to characterize the quality of the recovered paper.
Several methods have been proposed for quantifying stickies in the pulp slurry at the wet end of the paper machine. Generally, these involve screening a sample of the pulp through a laboratory-screening device under controlled screening conditions to screen out macro stickies, namely stickies of a size greater than 0.10 millimeters (mm) or 0.004 inches. Smaller stickies, i.e., micro stickies, which are not deemed particularly deleterious, pass through the screen with the cellulose fibers in the pulp. The filtrate or rejects from the screen are transferred to a black filter paper. A coated paper is placed on top of the filter paper, and together they are heated and pressed under controlled conditions. When the coated paper is removed from the filter paper, the coating will be picked up by the stickies and create a contrast on the black filter paper, which allows measurement, visually or with an image analyzer, of the area and the number of the heat-set stickies. See, e.g., TAPPI Journal, Vol. 82, No. 2, February 1999, pages 143-151.
U.S. Pat. No. 5,823,677 discloses a method of detecting and identifying stickies based on infrared (IR) radiation. According to the disclosure, a temperature gradient will exist between the stickies and a reference, e.g., ambient temperature or a substrate containing the stickies, when a sample containing the stickies and the reference are allowed to cool or to warm from a first temperature to a second temperature over a controlled period of time. The temperature gradient or difference is determined by measurement of the infrared radiation of the stickies and the reference, preferably by use of a thermographic camera. Also according to the disclosure, a characteristic gradient exists for each type of stickie when allowed to lose or gain heat for the same amount of time. These characteristic gradients allow for identification of the type of stickie by the surface temperature. This gradient is a function of the rate of heat loss or gain that is uniquely associated with each type of stickie.
The method disclosed in U.S. Pat. No. 5,823,677 comprises the steps of providing a sample which contains both at least one stickie and a reference material at a first temperature; allowing the sample to change from the first temperature to a second temperature; scanning the sample with a means for measuring infrared radiation after the sample has reached the second temperature so as to sense the temperatures of said at least one stickie and the reference material; determining the temperature difference between said at least one stickie and the reference material; and identifying said at least one stickie by the temperature differential.
While the above-described methods of stickies detection are widely used in the paper industry, they do not provide adequate accuracy, precision, speed, or specific identification and quantification of material inclusions.
U.S. Pat. No. 5,842,150 discloses a method for qualitative and quantitative determination of the organic content in pulp, paper and effluents from pulp and paper mills using ultraviolet, visible, near-infrared and infrared energy techniques, including Raman Spectroscopy. Laser Raman Spectroscopy has also been used for quantifying lignin content in pulp samples.
It is an object of the present invention to provide a method of and apparatus for detecting the presence of contaminant materials in pulp and paper with accuracy, speed and precision, and with specific identification and quantification of all or selected ones of the constituents, contaminants in the pulp or paper. It is noted that as used herein the term xe2x80x9ccontaminantsxe2x80x9d is intended to broadly refer to substances whose presence are undesirable in paper or pulp products, with examples including but not limited to sclereids, shives, stickies, foreign substances, and the like.
It is in particular an object of the invention to provide a method of and apparatus for detecting the presence of sclereids, shives, and the like in pulp and paper, and the presence of stickies and the like in pulp and paper made in whole or in part from recycled stock, and to do so with greatly enhanced speed, accuracy and precision, and with specific identification and quantification of each stickie, sclereid, and shive substance present in the pulp or paper.
A further object of the invention is to utilize laser Raman spectroscopic technology, in combination with computer science, to provide precise identification and quantification of the constituents and contaminants in pulp or paper, especially pulp and paper made from or containing recycled stock.
A further object of the invention is to provide for high-speed laser Raman spectroscopic scanning of pulp and paper samples, transmission of Raman images of the constituents and/or contaminants in the samples to a computer, comparison of the transmitted images to a library of Raman fingerprints in computer memory, and communication to paper-making personnel of the identity, quantity and location in the sample of all and/or selected ones of the constituents and/or contaminants present in the sample.
Raman spectroscopic technology is capable of producing a distinctive xe2x80x9cimagexe2x80x9d of each substance submitted for spectroscopic examination and each image is different from the images of all other substances. It is noted that as used herein, the term xe2x80x9cimagexe2x80x9d as used with respect to spectroscopic technology is intended to broadly refer to data output from a spectrometer with or without subsequent processing. By way of example, an xe2x80x9cimagexe2x80x9d may comprise a spectrum or other imagery, fingerprint, vector data, attribute identifier or other signal by which one substance may be distinguished from another by Raman technology. In effect, the technology produces a fingerprint individual to each substance, just as the fingerprint of each human being is distinctive and different from the fingerprints of all other human beings. While the differences may be slight, there is nevertheless sufficient difference to distinguish one human fingerprint from all other human fingerprints. So too with Raman spectroscopy. Though the differences between Raman images may be slight, the differences are sufficient to distinguish each substance from all other substances, i.e., each substance has its own Raman fingerprint.
Raman spectroscopy does not require a color or black and white contrast between the constituents of and contaminants in a pulp or paper sample and therefore is not limited to the meager results of prior art techniques. Moreover, it is not limited to simply distinguishing contaminants from cellulose, it is capable of specifically identifying each contaminant substance and distinguishing varieties of contaminants from one another.
In accordance with the present invention, Raman technology is employed, first, to distinguish foreign substances, e.g., contaminants and other undesirable substances, from desired constituents of pulp or paper, primarily cellulose fibers, and second, to identify each of the foreign substances, to distinguish the foreign substances one from the other and to quantify the amount of each of the contaminants in the pulp or paper sample.
The apparatus employed pursuant to the invention to achieve the foregoing objectives is comprised principally of a Raman spectrograph including a Raman spectroscopic probe and a coherent light source for feeding high-intensity laser light to the probe. The probe contains optical assemblies for concentrating coherent light onto a pulp or paper sample and for receiving back scattered light from the sample and feeding back scattered light images to the spectrograph which disperses the light to generate a spectrum for transmission to a computer and analysis. The sample is mounted on a scanning table. The invention further comprises translating means for causing relative translation between the scannable surface and the spectrometer. Preferably, the translating means comprise a translating device such as a motor, gears, drives and the like for translating the scanning table along the X and Y-axes of the sample. The translating device is capable of high-speed movement with substantially instantaneous stop and go characteristics and is capable of being programmed through a controller for movement in pre-selected sample-scanning patterns. Control means are provided for controlling the translating means as desired. Preferred control means comprise a processor based controller, such as a computer or the like.
The movements of the scanning table are coordinated with energization of the coherent light source via the computer. Scanning of the sample may be continuous, via continuous scan or stroboscopic circuitry, or intermittent, via trigger scan circuitry, so that the coherent light source is triggered on at each pause in the intermittent travel of the scanning table and sample. The scan may be a simple back and forth scan of most or all of the surface of the sample, or the scan may be predicated on theories of statistical analysis for producing a statistically-acceptable Raman examination of the sample.
The method of the invention, in a first aspect, comprises establishment of a library of Raman spectrographic fingerprints of the constituents and/or the contaminants or other substances anticipated to be present in the pulp or paper samples to be examined. Each constituent and/or contaminant and/or other substance is identified and subjected to Raman spectroscopic examination and its identity and spectroscopic vector or Raman fingerprint inputted to computer memory.
As scanning of a sample occurs, the computer compares the Raman images transmitted to it by the spectrograph to the library of Raman fingerprints stored in computer memory and, in synchronism with the movements of the scanning table and the sample, generates a map of the sample illustrating the location and area of contaminants, especially the location and area of stickies, shives, and sclereids. In addition, the computer identifies, computes and communicates, via X-Y coordinates, the location, identity and amount of each of the contaminants. If a foreign substance in the sample is discerned by the Raman spectroscope, and its Raman image cannot be matched with a Raman fingerprint in memory, the substance is reported simply as an unknown. Since the location (X-Y coordinates) and area of the unknown has been identified on the sample map, the substance can be removed from the sample, subjected to physical and/or chemical analysis, and its identity and Raman fingerprint added to the Raman fingerprint library.
By virtue of the above-described method and apparatus, the identity and quantity of constituents and/or contaminants and/or foreign substances in pulp and paper, particularly the identity and amount of each variety of sticky, sclereid and/or shive present in pulp or paper, can be speedily determined and communicated to paper makers and merchants for purposes of enhancing control over the papermaking process and for purposes of ascertaining the print quality and other characteristics of the paper produced.
The foregoing and other objects and advantages of the invention will become apparent to those of reasonable skill in the art from the following detailed description as considered in conjunction with the accompanying drawings.